114 lines
3.5 KiB
C
114 lines
3.5 KiB
C
/*
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* Copyright (C) 2021 Alyssa Rosenzweig <alyssa@rosenzweig.io>
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*
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* Permission is hereby granted, free of charge, to any person obtaining a
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* copy of this software and associated documentation files (the "Software"),
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* to deal in the Software without restriction, including without limitation
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* the rights to use, copy, modify, merge, publish, distribute, sublicense,
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* and/or sell copies of the Software, and to permit persons to whom the
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* Software is furnished to do so, subject to the following conditions:
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*
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* The above copyright notice and this permission notice (including the next
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* paragraph) shall be included in all copies or substantial portions of the
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* Software.
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*
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* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
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* THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
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* SOFTWARE.
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*/
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#ifndef __AGX_MINIFLOAT_H_
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#define __AGX_MINIFLOAT_H_
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#include <math.h>
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#include "util/macros.h"
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/* AGX includes an 8-bit floating-point format for small dyadic immediates,
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* consisting of 3 bits for the exponent, 4 bits for the mantissa, and 1-bit
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* for sign, in the usual order. Zero exponent has special handling. */
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static inline float
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agx_minifloat_decode(uint8_t imm)
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{
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float sign = (imm & 0x80) ? -1.0 : 1.0;
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signed exp = (imm & 0x70) >> 4;
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unsigned mantissa = (imm & 0xF);
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if (exp)
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return ldexpf(sign * (float) (mantissa | 0x10), exp - 7);
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else
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return ldexpf(sign * ((float) mantissa), -6);
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}
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/* Encodes a float. Results are only valid if the float can be represented
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* exactly, if not the result of this function is UNDEFINED. signbit() is used
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* to ensure -0.0 is handled correctly. */
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static inline uint8_t
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agx_minifloat_encode(float f)
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{
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unsigned sign = signbit(f) ? 0x80 : 0;
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f = fabsf(f);
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/* frac is in [0.5, 1) and f = frac * 2^exp */
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int exp = 0;
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float frac = frexpf(f, &exp);
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if (f >= 0.25) {
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unsigned mantissa = (frac * 32.0);
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exp -= 5; /* 2^5 = 32 */
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exp = CLAMP(exp + 7, 0, 7);
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assert(mantissa >= 0x10 && mantissa < 0x20);
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assert(exp >= 1);
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return sign | (exp << 4) | (mantissa & 0xF);
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} else {
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unsigned mantissa = (f * 64.0f);
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assert(mantissa < 0x10);
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return sign | mantissa;
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}
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}
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static inline bool
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agx_minifloat_exact(float f)
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{
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float f_ = agx_minifloat_decode(agx_minifloat_encode(f));
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return memcmp(&f, &f_, sizeof(float)) == 0;
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}
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#ifndef NDEBUG
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static inline void
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agx_minifloat_tests(void)
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{
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/* Decode some representative values */
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assert(agx_minifloat_decode(0) == 0.0f);
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assert(agx_minifloat_decode(25) == 0.390625f);
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assert(agx_minifloat_decode(135) == -0.109375f);
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assert(agx_minifloat_decode(255) == -31.0);
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/* Verify exactness */
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assert(agx_minifloat_exact(0.0f));
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assert(agx_minifloat_exact(0.390625f));
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assert(agx_minifloat_exact(-0.109375f));
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assert(agx_minifloat_exact(-31.0));
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assert(!agx_minifloat_exact(3.141f));
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assert(!agx_minifloat_exact(2.718f));
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assert(!agx_minifloat_exact(1.618f));
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/* Check that all values round trip */
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for (unsigned i = 0; i < 0x100; ++i) {
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float f = agx_minifloat_decode(i);
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assert(agx_minifloat_encode(f) == i);
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assert(agx_minifloat_exact(f));
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}
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}
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#endif
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#endif
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